1. Field of the Invention
The present invention relates to a hydrogen generation system that is used to produce a hydrogen-rich reformed gas to be supplied to a fuel cell, and a fuel cell system having such a hydrogen generation system.
2. Related Art
Fuel cells require a supply of a hydrogen-rich gas to produce electricity. One conventional example of a hydrogen generation system that generates gas for such purposes includes a coiled tubing evaporator in a descending spiral where steam is generated for reformation (see, for example, Japanese Patent Laid-Open No. 2000-281311). FIG. 5 is a cross-sectional view which schematically shows a configuration of this conventional hydrogen generation system. As shown in FIG. 5, the conventional hydrogen generation system comprises a hollow cylindrical reformer 1 and a burner 2A. The reformer 1 is filled with catalyst particles or pellets. The burner 2 functions as a burning unit to heat the reformer 1. The above-mentioned coiled tubing 3 is placed inside a combustion cylinder 2B that is arranged within the reformer 1 over the burner 2A. The outlet end of the coiled tubing 3 is connected to a steam supply tubing 4. The steam supply tubing 4 is connected to a feed supply tubing 7 which becomes a mixed gas flow tubing 8. The mixed gas flow tubing 8 is connected to a mixed gas chamber 9 that is disposed over the combustion cylinder 2B. A reformed gas passage 5 is fitted around the reformer 1 for the flowing of the reformed gas. A combustion gas passage 6 is fitted around the reformed gas passage 5.
In the conventional hydrogen generation system having the above-mentioned configuration, water Y for the reforming process flows into the hydrogen generation system through the upper portion of the coiled tubing 3 and is heated by combustion gas when it runs through the tubing. The heated water is converted into a gas-liquid two-phase flow state and then into steam. The steam is fed to the steam supply tubing 4. The steam that is supplied to the steam supply tubing 4 is mixed with a feed material X flowing through the feed supply tubing 7. The mixture is then supplied to the mixed gas chamber 9 via the mixed gas flow tubing 8. The steam and the feed material X in the mixed gas chamber 9 are then supplied to the reformer 1 where they are reformed into a reformed gas by steam reformation. The reformed gas is exhausted to the outside through the reformed gas passage 5. The combustion gas generated by the burner 2A is used to heat the coiled tubing 3 and the reformer 1 and is then exhausted to the outside through the combustion gas passage 6.
However, the above-mentioned conventional hydrogen generation system has the problem of significant release of heat to the surrounding due to the outermost placement of the combustion gas passage 6, lowering the thermal efficiency of the system.
Furthermore, the steam generated in the coiled tubing 3 flows through the steam supply tubing 4 which runs inside the system to the reformer 1. The steam supply tubing 4 dissipates a large amount of heat, which further lowers the thermal efficiency.
In addition, the hot combustion gas heats the coiled tubing 3. This tends to cause a so-called “dried out” state under which no water is present within the coiled tubing 3, with the possibility of ebullition that causes intermittent evaporation. The steam expands rapidly through change of state from a liquid to a vapor. This suddenly increases resistance in the tubing. If water comes to ebullition again and again, the supply pressure of the water Y fluctuates greatly. This results in fluctuation in supply amount of the water Y and, in turn, fluctuation in supply amount of steam. The fluctuation in amount of steam for the catalysis in the reformer 1 exhibits problems with a higher tendency to cause fluctuation in the level of carbon monoxide (CO) in the reformed gas. Varying flow rate of steam causes fluctuation in supply pressure of the feed material X because the steam flowing through the steam supply tubing 4 is mixed with the feed material X and is then supplied to the reformer 1. This fluctuates the flow rate of the feed material X, varying the flow rate of the reformed gas to the fuel cell. Such variation results in unstable power generation in the fuel cell.
The present invention was made with respect to the above-mentioned circumstances, and an object thereof is to provide a hydrogen generation system with which thermal efficiency increases, the CO level in the reformed gas stabilizes, and a fixed supply amount of the reformed gas can be achieved.